Optical Nanocircuits and Nanoantennas

Clusters of nanoparticles can be designed to support collective optical resonances and guide the flow of displacement current induced by an external excitation. In this context, I have investigated the possibility to transplant relevant functionalities from low-frequency antenna engineering and circuit theory to the optical frequency range, in which the optical displacement current supported by the nano-structures takes the role of the conduction current in their low-frequency counterparts [1-4]. In particular, I have shown that relatively complex nanostructures can be designed, assembled and operated by accurately positioning a number of metallic and dielectric nanoparticles acting as modular lumped circuit elements (figure on the left), in direct analogy with electrical circuits [4]. These results represent an important step towards extending the powerful modular design tools of electronic circuits into nanophotonic systems.

Based on similar nanocircuit concepts, with my colleagues at UT Austin, I have shown for the first time that giant magnetic response at optical frequencies (figure below) can be achieved by designing rings of plasmonic nanoparticles (the optical equivalent of split-ring resonators) with controlled structural asymmetries [5-7]. This represents an important scientific breakthrough, as it overcomes the long-held belief that magnetism is inherently weak at optical frequencies, paving the way toward the realization of optical magnetic metamaterials and enhanced magnetic light-matter interactions at the nanoscale.

Finally, I am currently investigating the possibility to use suitably designed optical nanoantennas to realize multi-channel optical wireless links at the nanoscale, mimicking the functionalities of multiple-input multiple-output (MIMO) RF antenna systems at frequencies for which bandwidths and speed-rates may be increased by orders of magnitude.

Optical magnetism based on rings of plasmonic nanoparticles. (a) A split-ring resonator realizes a resonant circulation of conduction current over a subwavelength footprint, supporting a strong magnetic response without magnetic materials. This idea can be transplanted at optical frequencies based on rings of plasmonic nanoparticles, in which the displacement current takes the role of the conduction current. From Ref. [5]. (b) Multipolar contributions to the total SCS, for a four-particle nanoring assembled with AFM nano-manipulation (AFM image in the inset). Dashed and solid lines correspond to the case of symmetric and slightly asymmetric nanorings, respectively. Small structural asymmetries efficiently boost the magnetic response (blue line). (c) Magnetic field intensity distribution at the magneto-electric Fano resonance in (b). White arrows indicate the induced dipole moments in the particles. From Ref. [7].